2,2,6-Trimethyl-cyclohex-5 -en-1,4-dione and homologues thereof, are useful as starting materials in the synthesis of certain carotenoids (see e.g. O. Isler "Carotenoids" Birkhauser Verlag, Basel, 1971, page 130) and are also useful in the fields of flavouring and perfumery (see e.g. U.S. Pat. No. 3,380,456). Several methods are known for the synthesis of such compounds, usually starting from isophorone, i.e. 3,3,5-trimethyl-cyclohex-5-en-1-one, which is commercially available at a low price and in practically unlimited quantities. One such method, described in Tetrahedron Suppl., 8, 1-7 (1966), Helv. Chim. Acta, 39, 2041 (1956) and U.S. Pat. No. 2,917,539, is summarized in the following reaction scheme: ##STR2##
However, this synthetic route suffers from the disadvantages of producing low yields of the desired diketone, of comprising several successive reaction steps, and of requiring the use of relatively expensive reagents, so that it is unsuitable for use on an industrial scale.
In order to provide an industrially more advantageous synthetic route, we have attempted to prepare 2,2,6-trimethyl-cyclohex-5-en-1,4-dione by oxidizing 4-bromo-3,5,5-trimethyl-cyclohex-2-en-1-one with nitropropane or with a tertiary amine oxide such as trimethylamine oxide. However, instead of the desired diketone, this reaction yielded, 2,4,4-trimethyl-cyclohex-5-en-6-ol-1-one, i.e. the compound of formula ##STR3## (see German Offenlegungsschrift No. 2,202,066).
The known method for the direct oxidation of an allylic methylene group to give the corresponding carbonyl derivative, by means of actinic irradiation in an aqueous medium in the presence of N-bromosuccinimide (see e.g. Chem. Comm., 1969, 1220), proved unsuccessful when applied to isophorone. Other known oxidation methods were also unsuccessful, including those described in J. Am. Chem. Soc. 79, 6308 (1957), J. Am. Chem. Soc. 83, 2952, (1961), J. Org. Chem. 33, 3566, (1968), Tetrahedron Letters 1972, 1823, and Liebigs Ann. Chem. 627,237, (1959): all of these methods either did not produce any appreciable amount of the desired ketone, or gave yields too poor for practical utility.